1. Field of the Invention
The present invention relates to a substrate for information recording media and a manufacturing method thereof, and an information recording medium, and more specifically to a substrate for information recording media for use as a disk substrate in a hard disk drive or the like and a manufacturing method thereof, and an information recording medium such as a magnetic disk, an optical magnetic disk or an optical disk.
2. Description of the Related Art
In recent years, there has been remarkable progress in information technology, and development of various types of information recording medium for storing information such as magnetic disks, optical magnetic disks and optical disks has been carried out with vigor.
In a hard disk drive (HDD), recording and playback of information are carried out by means of a magnetic head flying over a data zone formed on a magnetic disk substrate. Known driving methods used include a CSS (contact start/stop) method and a ramp load method.
In the CSS method, a CSS zone in which uniform minute undulations of height several tens of nm are formed is provided along the inner periphery or the outer periphery of the magnetic disk substrate. The magnetic head flies over the data zone of the magnetic disk substrate while the magnetic disk substrate is rotating, and slides over the CSS zone of the magnetic disk substrate when the magnetic disk substrate stops or starts up.
In the ramp load method, the magnetic head flies over the magnetic disk substrate while the magnetic disk substrate is rotating, and is stored in a predetermined storage position when the magnetic disk substrate stops.
In both the CSS method and the ramp load method, while the magnetic disk substrate is rotating, the magnetic head is thus raised up slightly from the magnetic disk substrate, and flies over the surface of the magnetic disk substrate with a gap (hereinafter referred to as the “flying height”) of several tens of nm maintained between the magnetic head and the surface of the magnetic disk substrate.
In an HDD, it is necessary to prevent the magnetic head from being subjected to excessive resistance in the case that the magnetic head contacts the magnetic disk substrate while flying over the magnetic disk substrate. To this purpose, art in which a large number of minute projections referred to collectively as “texture” are formed on the surfaces of the magnetic disk substrate has been implemented from hitherto, and has been made fit for practical use.
Specifically, known art for forming a large number of minute undulations on the surfaces of a magnetic disk substrate includes art in which etching is carried out using a fluorine-containing liquid or hydrogen fluoride gas, thus forming a large number of minute undulations on a glass substrate (Japanese Laid-open Patent Publication (Kokai) No. 64-42025), art in which a glass substrate is subjected to crystallization treatment, then mirror-polishing is carried out, and then etching is carried out using a liquid prepared by adding sulfuric acid or ammonium fluoride to hydrofluoric acid, thus forming a large number of minute undulations on the glass substrate (Japanese Laid-open Patent Publication (Kokai) No. 7-296380), art in which ultrafine particles are applied onto a glass substrate, then dry etching is carried out, and then the ultrafine particles are removed, thus forming a large number of minute undulations on the glass substrate (Japanese Laid-open Patent Publication (Kokai) No. 8-249654), and art in which projections are formed on the surfaces of a glass substrate by irradiating with a laser beam (Japanese Laid-open Patent Publication (Kokai) No. 7-182655, Japanese Laid-open Patent Publication (Kokai) No. 9-194229).
With current demands to increase the recording density of HDDS, there have been demands to further reduce the flying height. To this purpose, it is necessary to reduce the projection height of the minute undulations formed on the surfaces of the glass substrate, and moreover to suppress variation in the projection density, thus enabling head crashes and thermal asperity to be avoided. Moreover, to cope with increases in the recording density, it is necessary to improve the degree of cleanliness of the surfaces of the glass substrate, and yet there have also come to be demands to manufacture glass disk substrates at a low cost about the same as that of conventional aluminum disk substrates.
Amid this situation, these days the manufacture of magnetic disk substrates is widely carried out by subjecting a glass substrate to precision polishing, then forming minute undulations on the surfaces of the glass substrate by carrying out etching, and then subjecting the glass substrate to chemical strengthening treatment.
FIG. 8 is a flowchart showing a conventional method of manufacturing a magnetic disk substrate (see, for example, Japanese Laid-open Patent Publication (Kokai) No. 2000-132829; hereinafter referred to as “the first prior art”). In the first prior art, a starting material glass plate that has been manufactured in a starting material glass plate manufacturing step 101 is cut into a donut shape in a disk processing step 102, then the inner and outer peripheral surfaces of the starting material glass plate are processed to predetermined dimensions in an edge processing step 103, next the surfaces of the resulting glass substrate are polished in a surface polishing step 104, then the surfaces of the glass substrate are washed using silicofluoric acid or the like and thus etched in a surface washing step 105, then the glass substrate is strengthened in a chemical strengthening treatment step 106, and then, in a finishing washing step 107, the glass substrate is washed by immersing in sulfuric acid heated to 40° C. while irradiating with ultrasound if necessary, thus etching the glass substrate and hence removing residual foreign matter such as iron powder stuck to the glass substrate. Through these steps, a magnetic disk substrate is manufactured.
In the surface polishing step 104, the polishing of the surfaces of the glass substrate is carried out in three steps, i.e. a rough polishing step 104a, a pre-polishing step 104b, and a precision polishing step 104c. 
Moreover, as other prior art, a method of manufacturing a magnetic disk substrate is also known in which, after chemical strengthening treatment has been carried out on a glass substrate to form a strengthened layer, polishing is carried out on the main surfaces (recording surfaces) of the glass substrate, thus removing the strengthened layer from the main surfaces of the glass substrate while leaving the strengthened layer behind on only the inner and outer peripheral edge surfaces of the glass substrate (Japanese Patent Application Laid-open No. 2000-207730; hereinafter referred to as “the second prior art”).
Furthermore, a method of manufacturing a magnetic disk substrate has also been proposed in which chemical strengthening treatment is carried out to form a strengthened layer of thickness at least 20 μm, preferably at least 50 μm, then at least 10 μm of the strengthened layer is polished away by precision polishing, resulting in a state in which at least 10 μm of the strengthened layer remains, and then de-alkalization treatment is carried out in this state (Japanese Patent Application Laid-open No. 2000-128583; hereinafter referred to as “the third prior art”).
However, in the first prior art described above, even though the variation in the projection height of the minute undulations can be suppressed to be within a certain range through the etching in the surface washing step 105 after the precision polishing step 104c, the surface shape changes due to the subsequent chemical strengthening treatment step 106, and also changes due to the subsequent washing in the finishing washing step 107. There is thus a problem that the uniformity of the projection height of the minute undulations on the glass substrate is lost, and moreover the variation in the projection density increases. Specifically, in the chemical strengthening treatment step 106, compressive stress is imparted to the main surfaces of the glass substrate through ion exchange between Na+ and K+ in the molten salt, thus strengthening the substrate. However, due to disuniformity in the composition of the molten salt and degradation of the molten salt over time, the surface shape (minute undulations) changes when the compressive stress is imparted. Moreover, after the chemical strengthening treatment, compressed layers produced through the chemical strengthening treatment and compressed layers produced through the precision polishing 104c are intermixed on the substrate surfaces. In the finishing washing step 107, as described above the glass substrate is immersed in sulfuric acid heated to 40° C. to remove foreign matter by etching; however, at the compressed layers produced through the chemical strengthening treatment and the compressed layers produced through the precision polishing 104c, the degree of etching by the treatment liquid differs, and hence the surface shape changes between before and after the finishing washing step 107. There is thus a problem that, compared with the surface shape before the chemical strengthening treatment step 106, after the finishing washing step 107 the uniformity of the projection height of the minute undulations has been lost, and the variation in the projection density has increased, both across a single glass substrate, and between a plurality of glass substrates.
Moreover, in the second prior art, the strengthened layer formed on each of the main surfaces of the glass substrate is completely removed, and hence there is a problem that polishing scrap (i.e. matter removed from the glass substrate) produced through the polishing is discharged in a large amount, leading to an increase in the amount of industrial waste, and resulting in an increase in the manufacturing cost.
Moreover, in the third prior art, it is necessary to make at least 10 μm of the strengthened layer on each of the main surfaces of the glass substrate remain behind even after the main surfaces have been polished, and hence the starting material glass plate must be formed to be extra thick. Moreover a large amount of polishing scrap is discharged through the polishing, and hence, as with the second prior art, there is a problem that this leads to an increase in the amount of industrial waste, and results in an increase in the manufacturing cost.